Polymeric fibers have been known essentially since the beginnings of commercial polymer development. The production of polymer fibers from polymer films is also well known. Typically, molten polymer is extruded through a die or small orifice in a continuous manner to form a continuous thread. The fiber can be further drawn to create an oriented filament with significant tensile strength. Fibers created by a traditional melt spinning process are generally larger than 15 microns. Smaller fiber sizes are impractical because of high melt viscosity of the molten polymer. Fibers with a diameter less than 15 microns can be created by a melt blowing process. However, the resins used in this process are low molecular weight and viscosity rendering the resulting fibers very weak. In addition, a post spinning process such as length orientation cannot be used.
Orientation of crystalline polymeric films and fibers has been accomplished in numerous ways, including hot drawing, melt spinning, melt transformation (co)extrusion, solid state coextrusion, gel drawing, solid state rolling, die drawing, solid state drawing, and roll-trusion, among others. Each of these methods has been successful in preparing oriented, high modulus polymer fibers and films. Most solid-state processing methods have been limited to slow production rates, on the order of a few cm/min. Methods involving gel drawing can be fast, but require additional solvent-handling steps. A combination of rolling and drawing solid polymer sheets, particularly polyolefin sheets, has been described in which a polymer billet is deformed biaxially in a two-roll calender then additionally drawn in length (i.e., the machine direction). Methods that relate to other web handling equipment have been used to achieve molecular orientation, including an initial nip or calender step followed by stretching in both the machine direction or transversely to the film length.
The production of macroscopic fibers from films has been established. Liberating fibers from oriented, high-modulus polymer films, particularly from high molecular weight semicrystalline films, has been accomplished in numerous ways, including abrasion, mechanical plucking by rapidly-rotating wire wheels, and impinging water jets to slit the film. Water jets have been used extensively to cut films into flat, wide continuous longitudinal fibers for strapping or reinforcing uses.
Pennings et. al. in “Mechanical properties and hydrolyzability of Poly(L-lactide) Fibers Produced by a Dry-Spinning Method”, J. Appl. Polym. Sci., 29, 2829-2842 (1984) described fibers with a fibrillar structure by solution spinning using chloroform in the presence of various additives (camphor, polyurethanes) followed by hot drawing. These fibers showed good mechanical properties and improved degradability in vitro with the fibrillar structure speeding up the hydrolysis of the fiber. The inherent disadvantage of this process is the use of chlorinated solvents in the spinning process.
Microfibers with a diameter of 1 micrometer and a round cross section have also been produced by electrospinning. The electrospinning technique also suffers from the disadvantage of using a chlorinated solvent and has low production speeds.
WO 95/23250 discloses a process for preparing biodegradable fibrils from polylactide where a polymer solution is precipitated into a non-solvent. The fibrils can be dried and formed into a biodegradable nonwoven article.
U.S. Pat. No. 6,111,060 (Gruber et al.) discloses the use of melt stable polylactides to form nonwoven articles via melt blown and spunbound processes. These fibers have low orientation and have generally low tensile strength. In addition, the fibers have a round cross sectional area comparable to traditional textile fibers.
WO 9824951 discloses the production of multicomponent fibers for nonwovens comprising two different polylactides.